1. Field of the Invention
The present invention relates to an optical fiber laser and a method of harmonic mode locking utilizing thereof, and more particularly to an optical fiber laser and a method of harmonic mode locking utilizing thereof which is capable of harmonic mode locking by inserting an opto-acoustic variable filter into the optical fiber laser which uses an Erbium Doped fiber as a gain medium.
2. Description of the Related Art
In advance to explain the present invention, the problem of the conventional method will be described with reference to FIGS. 1 and 2, more in detail. In order to obtain a high repetition rate at an optical fiber laser, a structure representing an active mode locking is shown in FIG. 1, and a structure representing a passive mode locking is shown in FIG. 2 according to the prior art. In FIGS. 1 and 2, the reference numeral 11 denotes a 50:50 optical fiber directional coupler or fiber coupler, the reference numeral 12 a wavelength division multiplier fiber coupler, the reference numeral 21 a polarization controller, the reference numeral 32 an optical isolating polarizer, the reference numeral 42 an Erbium doped optical fiber, the reference numeral 43 a single mode optical fiber for general communication, the reference numeral 51 980 nm optical pumping laser diode, and the reference numeral 81 an optical modulator, respectively.
In order to oscillate an ultrashort optical pulse and obtain a high repetition rate at the optical fiber laser according to the prior art, it can be obtained from the active mode locking by modulating the amplitude or the phase of the optical laser utilizing an high speed optical modulator, as can be referred to in FIG. 1. In this structure, the high repetition rate is determined depending on the frequency applied to the optical modulator 81. Therefore, the high repetition rate which oscillates the ultra optical pulse has an limit according to the frequency applied to the optical modulator 81.
In the meantime, a optical pulse source having a high repetition rate can also be obtained in the structure of the passive mode locking, as shown in FIG. 2. In this structure, the high repetition rate is obtained by a desired physical phenomenon in the laser resonator, which occurs the harmonic mode locking in the frequency corresponding to the higher order of the longitudinal mode in the laser resonator. However, in this structure, the output of the optical source is unstable. Therefore, it is difficult to obtain a normal repetition rate of the optical pulse.
Therefore, it is an object of the present invention to overcome the above disadvantages and provide an optical fiber laser and a method of harmonic mode locking utilizing the optical fiber laser, which is capable of harmonic mode locking by inserting an acousto-optic modulator between a Faraday rotator mirror and a 3dB directional coupler in the structure utilizing the non-linear amplifying loop mirror and Faraday rotator mirror.